1. Field of the Invention
The present invention relates to oscillators and voltage controlled oscillators, and, more particularly, relates to an oscillator and a voltage controlled oscillator which have a surface acoustic wave resonator.
2. Description of the Related Art
FIG. 12 schematically illustrates a circuit diagram showing the configuration of a conventional Colpitts voltage controlled oscillator (VCO).
In FIG. 12, the Colpitts VCO includes an oscillation circuit 60, a buffer circuit 66, and a coupling capacitor 67 for coupling the oscillation circuit 60 and the buffer circuit 66. The oscillation circuit 60 includes a surface acoustic wave (SAW) resonator 51, an NPN transistor 52, capacitors 53 to 56, a varicap 57, and resistance elements 58 and 59. The buffer circuit 66 includes an NPN transistor 61 and resistance elements 62 to 65.
The oscillating frequency of the oscillation circuit 60 is determined by the resonance frequency of the SAW resonator 51 and the capacitance of the oscillation circuit 60. The oscillating frequency of the oscillation circuit 60 varies because the capacitance of the varicap 57 changes in accordance with change of the input voltage VI to the oscillation circuit 60. Therefore, an output signal VO having a desired frequency can be obtained by adjusting the input voltage VI. This oscillation circuit is widely used as a reference-signal generator for a television tuner, a portable communication device, and the like.
FIG. 13 schematically illustrates a circuit diagram of a conventional differential-LC VCO. In FIG. 13, the differential-LC VCO includes a parallel LC circuit 71, resistance elements 74 and 75, NPN transistors 76 and 77, and a voltage controlled current source 78. The parallel LC circuit 71, having an inductor 72 and a capacitor 73 connected in parallel, is connected between the collectors (nodes N71 and N72, respectively) of the NPN transistors 76 and 77. The collectors of the NPN transistors 76 and 77 are connected to a power line of a source electrical potential Vcc via resistance elements 74 and 75, respectively. The bases of the NPN transistors 76 and 77 are connected to the collectors of the NPN transistors 77 and 76, respectively. The emitters of the NPN transistors 76 and 77 are each grounded, via the voltage controlled current source 78, to a ground line of a ground potential GND.
The oscillating frequency of this differential-LC VCO is determined by the parallel resonant frequency of the parallel LC circuit 71 and the internal capacitance of each of the NPN transistors 76 and 77 which are connected in parallel with the parallel LC circuit 71. Because the capacitance of each of the NPN transistors 76 and 77 is varied by the values of the currents passing through the NPN transistors 76 and 77, the oscillating frequency of the VCO can be adjusted by causing an external control voltage to control the current of the voltage controlled current source 78. This circuit is utilized by an integrated circuit for television, and the like.
However, the Colpitts VCO shown in FIG. 12 has the following problems: because of the single ended configuration thereof, noise generated by the oscillation may produce adverse effects on other circuits via the power source; and conversely, a signal of the VCO may be modulated by high frequency noise coming through the power source. Furthermore, although it is intended to miniaturize the circuit by integrating the components of the circuit other than the SAW resonator 51, because there are many capacitance elements 53 to 56 and 67 which occupy more space. than the resistance elements or the transistors, the integration of those elements is difficult. Also, when each element is constructed as a discrete component, since many elements are required for the circuit, miniaturization of the circuit is difficult.
In the differential-LC VCO in FIG. 13, although a wire wound variable inductor having a high quality-factor (Q-factor) must be employed as the inductor 72 to obtain an accurate and stable oscillation, miniaturization of the VCO is difficult due to the large size thereof. Moreover, in order to obtain a desired frequency, the inductance of the variable inductor must be adjusted. Additionally, because the VCO requires a variable inductor, the inductance of which must be adjusted by a mechanical operation as a variable component, the reliability is low.
Accordingly, it is an object of the present invention to provide an oscillator and a voltage controlled oscillator which are highly resistant to noise, have accurate oscillation without adjustment, have high reliability, require less components, and are inexpensive and compact.
To this end, according to a first aspect of the present invention, there is provided an oscillator oscillating at a predetermined frequency. The oscillator includes a first inverting amplifier and a second inverting amplifier connected in antiparallel with each other, a surface acoustic wave resonator connected in parallel with one of said first inverting amplifier and said second inverting amplifier and a filter connected between said first inverting amplifier and said second inverting amplifier for blocking a direct-current signal while allowing a signal of said predetermined frequency to pass.
Since the direct current signal is blocked by the filter, the oscillating frequency of this oscillator is determined primarily by the parallel resonant frequency of the surface acoustic wave resonator.
This oscillator is not susceptible to the influence of external noise flowing via the power source and, conversely, is unlikely to convey noise to other circuits, compared to the Colpitts, due to a differential circuit employed therein. Also, the number of components of the oscillator is small, which makes miniaturization easy and is effective for reducing costs. Furthermore, because a capacitor having large capacitance is not used, it is possible to integrate a circuit. By integrating the entire circuit including the resonator, or the circuit other than the resonator, further miniaturization as well as further reduction in costs is possible. Effects of reduction in the number of components includes improving reliability of the oscillator.
Since, in the differential LC oscillator, when the Q-factor of the inductor is low, the Q-factor of the oscillator inevitably becomes low, which makes the stability of the oscillating frequency worse. On the other hand, since this oscillator uses a SAW resonator, accurate and highly stable oscillation can be obtained without adjustment. Moreover, since a variable component which needs mechanical adjustment is not used, reliability becomes high and deterioration with age becomes less.
According to a second aspect of the present invention, there is provided a voltage controlled oscillator which oscillates at a frequency in accordance with a control voltage. The voltage controlled oscillator includes a pair of differential transistors, in which a first electrode of one transistor is connected to a first electrode of the other transistor and an input electrode of one transistor is connected to a second electrode of the other transistor with regard to each of the transistors, a surface acoustic wave resonator connected between the second electrodes of the pair of differential transistors, a first capacitor coupled between the input electrode of at least one transistor of the pair of differential transistors and the second electrode of the other transistor, and a voltage controlled current source connected to the first electrodes of the pair of differential transistors for flowing currents having a value in accordance with the control voltage.
Because the first capacitor blocks the direct current, the oscillating frequency of this voltage controlled oscillator is determined primarily by the parallel resonant frequency of the surface acoustic wave resonator and the value of current flowing via the pair of differential transistors. This voltage controlled oscillator also enables the same effect as obtained by the invention according to a first aspect of the invention to be obtained.
The voltage controlled oscillator may further have a resistance element connected in parallel to the surface acoustic wave resonator. In this case, the variable frequency range is expanded.
The voltage controlled oscillator may further have an inductor connected in series to the surface acoustic wave resonator between the second electrodes of the pair of differential transistors. In this case, the variable frequency range is expanded.
The voltage controlled oscillator may further have an inductor connected in parallel to the surface acoustic wave resonator. In this case, the variable frequency range is expanded.
The voltage controlled oscillator may further have a second capacitor connected in series to the surface acoustic wave resonator between the second electrodes of the pair of differential transistors. In this case, the variable frequency range becomes narrowed, whereby oscillation becomes more stable.
The voltage controlled oscillator may further have a second capacitor connected in parallel to the surface acoustic wave resonator. In this case, the variable frequency range becomes narrowed, whereby oscillation becomes more stable.